git-gui: Run git-gc rather than git-repack.

Now that git 1.5.0-rc1 and later has a 'git gc' command which performs
all important repository management activites (including reflog pruning,
repacking local objects, unnecessary loose object pruning and rerere cache
expiration) we should run 'gc' when the user wants us to cleanup their
object database for them.

I think the name 'gc' is horrible for a GUI application like git-gui,
so I'm labeling the menu action 'Compress Database' instead.  Hopefully
this will provide some clue to the user about what the action does.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>